Non-aqueous electrolyte secondary batteries, which are typically represented by lithium ion secondary batteries, have a high electromotive force and a high energy density, and therefore a demand is increasing for lithium ion secondary batteries as a main power source for mobile telecommunication devices and portable electronic devices. Majority of the lithium ion secondary batteries currently on the market use a lithium composite oxide mainly composed of cobalt as the positive electrode active material (for example, LixCoO2 (x changes by charging and discharging of the battery)). However, in the case of the lithium composite oxide mainly composed of cobalt, because the cost of the cobalt compound used as the raw material is high, cost reduction is difficult.
In view of cost reduction, research and development have been carried out for various positive electrode active materials that can be a substitute for lithium cobalt oxide. Particularly, vigorous researches have been carried out for a lithium composite oxide composed mainly of nickel (for example, LixNiO2 (x changes by charging and discharging of the battery)).
In addition to the cost reduction, it is important to increase reliability of the lithium ion secondary battery. Upon charging, lithium composite oxides such as LixCoO2 and LixNiO2 contain Co4+ and Ni4+ that are highly reactive and high in valence. Due to this fact, under a high-temperature environment, the electrolyte decomposition reaction involving the lithium composite oxide is accelerated to generate gas in the battery, declining lifetime performance. Regarding the reactivity with the electrolyte under charged state, it is known that LixNiO2 is highly reactive with the electrolyte more than LixCoO2. Also, in lithium composite oxides such as LixCoO2 and LixNiO2, when oxidation numbers of Co and Ni are high, their crystal structures are unstable. Therefore, upon repeating charge and discharge cycle, their crystal structures are gradually broken, failing to achieve sufficient cycle performance, and declining lifetime performance.
To curb the decomposition reaction of the electrolyte, there has been proposed that a coating film comprising a specific metal oxide is formed on the positive electrode active material surface (Patent Documents 1 to 5). Also, there has been proposed that by incorporating a different element in the lithium composite oxide to form a solid solution to stabilize the crystal structure of the lithium composite oxide, cycle performance and storage characteristics under high temperature are improved (Patent Documents 6 to 8).    [Patent Document 1] Publication of Japanese Patent No. 3543437    [Patent Document 2] Japanese Laid-Open Patent Publication No. Hei 11-317230    [Patent Document 3] Japanese Laid-Open Patent Publication No. Hei 11-16566    [Patent Document 4] Japanese Laid-Open Patent Publication No. 2001-196063    [Patent Document 5] Japanese Laid-Open Patent Publication No. 2003-173775    [Patent Document 6] Japanese Laid-Open Patent Publication No. 2004-111076    [Patent Document 7] Japanese Laid-Open Patent Publication No. Hei 11-40154    [Patent Document 8] Japanese Laid-Open Patent Publication No. 2002-15740